A compensating circuit for a cold junction of a thermocouple includes a sign inverting amplifier, a power source and a summing amplifier. A thermo-electromotive force of the thermocouple, the output of the sign inverting amplifier and the voltage of the power source are applied to one input terminal of the summing amplifier through resistors. An output corresponding to the temperature of the hot junction of the thermocouple is obtained from the output terminal of the summing amplifier.
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1. A compensating circuit for a cold junction of a thermocouple, comprising a power source, a sign inverting dc amplifier having one input thereof connected to said power source, a heat-sensing resistor connected in a feedback path of said sign inverting amplifier and a summing amplifier having an input connected to the output of said sign inverting dc amplifier, a thermoelectromotive force generated from a thermocouple and the voltage of said power source are applied to said input of said summing amplifier through resistors, respectively, whereby an output corresponding to the temperature of the hot junction of said thermocouple is obtained from an output terminal of said summing amplifier.
2. A compensating circuit for a cold junction of a thermocouple according to
3. A compensating circuit for a cold junction of a thermocouple according to
4. A compensating circuit for a cold junction of a thermocouple according to
5. A compensating circuit for a cold junction of a thermocouple according to
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1. Field of the Invention
This invention relates to a compensating circuit for a cold junction of a thermocouple as a thermo-electro transducer and more particularly to a compensating circuit for a cold junction of a thermocouple which is stable in the wide range of the temperature.
2. Description of the Prior Art
In the conventional method for the compensation of the cold junction of the thermocouple, the cold junction is dipped into iced water at 0°C. The conventional method is highly accurate, but it has the disadvantage that it is not simple. An electrical method has been proposed for the compensation of the cold junction. However, its operation is unstable and not very accurate. For example, there is a Wheatstone bridge connected to the cold junction of the thermocouple. A copper resistor is connected in one line of the bridge. However, it is difficult to balance perfectly the bridge in the wide range of the temperature.
An object of this invention is to provide a compensating circuit for a cold junction of a thermocouple which can compensate electrically a cold junction.
Another object of this invention is to provide a compensating circuit for a cold junction of a thermocouple which can compensate simply and accurately the cold junction.
A further object of this invention is to provide a compensating circuit for a cold junction of a thermocouple which can operate stably in a wide range of the temperature.
A still further object of this invention is to provide a compensating circuit for a cold junction of a thermocouple in which a thermoelectromotive force of a thermocouple can be amplified.
The above, and other objects, features and advantages of the invention, will be apparent in the following detailed description of illustrative embodiments thereof which is to be read in connection with the accompanying drawings.
The FIGURE is a diagrammatic representation of a compensating circuit for a cold junction of a thermocouple according to one embodiment of this invention.
One embodiment of a compensating circuit for a cold junction of a thermocouple will be described with the drawing.
Referring to the drawing, TP represents a thermocouple, HC a hot junction and CC a cold junction. A resistor R1 and a power source -E are connected between a minus input terminal of a first operational amplifier OP1 and the ground. A copper resistor Rt is arranged in a feedback line of the first operational amplifier OP1. The copper resistor Rt detects the temperature of the cold junction CC of the thermocouple TP. A plus input terminal of the first operational amplifier OP1 is connected to the ground. Thus, the first operational amplifier OP1 is used as a sign-inverting DC amplifier. A minus input terminal of a second operational amplifier OP2 is connected to the thermocouple TP through a resistor R2, to the output terminal of the first operational amplifier OP1 through a resistor R3, and to the power source -E through a resistor R4. A resistor R5 is arranged in a feedback line of the second operational amplifier OP2. Thus, the second operational amplifier OP2 forms a summing amplifier together with the resistors R2, R3, R4 and R5. A potentiometer RV is connected to a plus input terminal of the second operational amplifier OP2. T represents an output terminal of the second operational amplifier OP2.
Next, operations of the compensating circuit for the cold junction of the thermocouple will be described.
If Vof0 is a conversion offset input voltage of the first operational amplifier OP1, the output voltage V1 of the first operational amplifier OP1 is represented by
V1 = Rt /R1 E + (1 + Rt /R1) Vof0 1.
If thermoelectromotive forces Eh and Ec are generated at the hot junction HC and at the cold junction CC, respectively, a thermoelectromotive force Eh - Ec is obtained from the thermocouple TP. Since the summing amplifier is formed by the resistors R2 - R5 and the second operational amplifier OP2, an output voltage Vout at the output terminal T is represented by ##EQU1## where Vof2 represents a conversion offset input voltage of the second operational amplifier OP2 and V2 a voltage applied to the plus input terminal of the second operational amplifier OP2. The resistance of the copper resistor Rt is represented by
Rt = Rto (1 +αt) (3),
where t represents the temperature of the copper resistor Rt, Rto the resistance of the copper resistor Rt at 0°C and α the temperature coefficient of the copper resistor Rt. The copper resistor Rt and the cold junction CC of the thermocouple TP are arranged in such a manner that the temperature of the cold junction CC of the thermocouple TP is equal to that of the copper resistor Rt.
From the equations (1), (2) and (3), the output voltage Vout at the output terminal T is represented by ##EQU2##
If the following relationship holds good in a permissible temperature range for the cold junction CC of the thermocouple TP;
Ec = βt (5),
the second term of the equation (4) amounts to zero without dependence on the temperature of the cold junction CC when the resistor R1, R2 or R3, or the voltage of the power source E is determined so as to fulfill the following relationship. ##EQU3## where β is a constant proper to the thermocouple.
Also the third term of the equation (4) amounts to zero without dependence on the temperature of the cold junction CC in such a manner that the input voltage V2 is adjusted by the potentiometer RV so as to put the output voltage Vout into zero volt at the temperature 0°C of the hot junction HC. Thus, the equation (4) is converted into ##EQU4##
It is preferable to use the resistors R1 - R5 the potentiometer RV and the power source -E stable against the temperature.
A platinum resistor may be used instead of the copper resistor. The first operational amplifier can be of an inexpensive normal type. The thermoelectromotive force Eh generated at the hot junction HC is very small. Therefore, it should be generally amplified several hundred times. When a chopper amplifier CA is connected between the resistor R2 and the cold junction CC, or to the output terminal T, at R5 /R2 ≈ 1 in the equation (7), the second operational amplifier can be inexpensive. According to this invention, the thermoelectromotive force Eh can be amplified several hundred times by the resistors R2 and R5, and the second operational amplifier OP2, where the second operational amplifier OP2 should have less thermal drift.
Although an illustrative embodiment of the invention has been described in detail herein with reference to the accompanying drawing, it is to be understood that the invention is not limited to this precise embodiment, and that various changes and modifications may be effected therein without departing from the scope or spirit of the invention as defined in the appended claims.
| Patent | Priority | Assignee | Title |
| 4120201, | Mar 18 1977 | TIME & TEMPERATURE CONTROLS CORP | Thermocouple temperature measurement circuit having cold junction compensation |
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| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Sep 20 1974 | Iwatsu Electric Co., Ltd. | (assignment on the face of the patent) | / |
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